How to calculate concrete for slabs, footings and post holes

Running short on a pour is one of the worst feelings on site. The truck is gone, the concrete you’ve placed is starting to go off, and the next load is an hour away. Ordering too much isn’t much better: you pay for concrete you then have to get rid of. Getting the number right takes ten minutes with a tape, a calculator and the formulas below.

This guide covers how to work out volume. It does not cover how thick a slab should be, how deep a footing should go or what strength of concrete to use. Those come from your plans, your engineer and the standards that apply where you work.

Work in metres from the start

Concrete is ordered in cubic metres (m³), so convert every measurement to metres before you multiply. Most site measurements are in millimetres, so divide by 1,000:

• 100 mm = 0.1 m
• 300 mm = 0.3 m
• 450 mm = 0.45 m

Mixing millimetres and metres in one sum is the most common way to end up with an answer that’s out by a factor of a thousand. If you need litres, 1 m³ is 1,000 litres.

The basic formula

Almost everything you pour is a box, a strip or a cylinder, and all three come back to one idea:

Volume = area of the shape × depth

Slabs and pads

A rectangular slab is length × width × thickness.

For an L-shaped or odd-shaped slab, split it into rectangles, work out each one, and add them together. Don’t try to do it in one go.

Strip footings

A strip footing is a long, thin rectangle: total length × width × depth. Measure the total length along the centre line of the footing. If you add up the outside edges of a rectangular building you’ll count the corners twice and over-order slightly, which is not a disaster but adds up on a big job.

If the slab has thickened edges or internal beams, work those out as separate strips and add them to the slab volume.

Round post holes

A round hole is a cylinder:

Volume = π × r² × depth

π (pi) is about 3.1416, and r is the radius, which is half the diameter. So for a 300 mm hole, r = 0.15 m, and r² = 0.15 × 0.15 = 0.0225.

Work out one hole, then multiply by the number of holes. Square holes are just length × width × depth.

Steps

Treat solid steps as a stack of blocks. The bottom block runs the full length of the steps, the next one is one tread shorter, and so on. Work out each block and add them up. A quick shortcut for solid steps with equal rises and treads: multiply rise × tread × (1 + 2 + 3, for however many steps there are) × width.

Worked examples

These are examples only, with round numbers chosen to make the arithmetic easy to follow.

Slab: 6.0 m × 4.0 m × 100 mm thick. 6.0 × 4.0 × 0.1 = 2.4 m³.

Strip footing: 20 m total centre-line length, 300 mm wide, 450 mm deep. 20 × 0.3 × 0.45 = 2.7 m³.

Post holes: eight holes, 300 mm diameter, 600 mm deep. One hole is 3.1416 × 0.0225 × 0.6 = 0.0424 m³. Eight holes is about 0.34 m³.

Steps: three solid steps, 1.2 m wide, 300 mm treads, 170 mm rises. 0.17 × 0.3 × (1 + 2 + 3) × 1.2 = 0.367 m³, so about 0.37 m³.

Element	Measurements	Net volume	With 10% added
Slab	6.0 × 4.0 × 0.1 m	2.40 m³	2.64 m³
Strip footing	20 × 0.3 × 0.45 m	2.70 m³	2.97 m³
Post holes	8 holes, 0.3 m dia × 0.6 m	0.34 m³	0.37 m³
Steps	3 steps, 1.2 m wide	0.37 m³	0.40 m³

Add a waste allowance

Your formula gives you the volume of a perfect shape. Real sites aren’t perfect. Ground is uneven, trenches get dug wider than planned, boxing bows out under load, and some concrete always stays in the chute, the pump line or the barrow.

A common rule of thumb is to add around 5% for a simple, well-formed pour on firm ground and up to around 10% for irregular ground, hand-dug holes or trenches, or a crew you haven’t poured with before. Adjust it from your own experience on similar jobs.

It’s worth checking depth in several places before you order. On the example slab above, being just 10 mm deeper than planned across the whole area is 6.0 × 4.0 × 0.01 = 0.24 m³ extra. That’s most of a 10% allowance gone before anything else goes wrong.

Premix bags: read the bag

For small jobs like a few post holes, bagged premix is often easier than a truck. The yield of a bag depends on the product and bag size, so check what’s printed on the bag or the manufacturer’s data sheet rather than going by a figure you remember.

As an example, one New Zealand manufacturer lists a yield of about 0.012 m³ for a 25 kg bag of its general concrete premix. On that figure, the eight post holes above (0.34 m³) would take 0.34 ÷ 0.012 = about 28.3 bags, so 29 bags before any waste allowance. Add your allowance and round up. A different product or bag size will give a different answer, which is exactly why you read the bag.

Once you get past a cubic metre or so, the number of bags, the mixing time and the effort usually make ready-mix the better choice. Do the sums both ways and compare.

Ordering ready-mix

When you ring the plant, have this ready:

• The volume, rounded up, with your waste allowance included
• The mix: strength and any other requirements, taken from your plans or specification. Don’t guess it.
• The site address and access: truck size, overhead wires, soft ground, steep driveways, and how close the truck can get
• Placement method: straight from the chute, barrows, or a pump. If you need a pump, book it at the same time.
• Timing: when you want the first truck, and the gap between loads on a bigger pour

Ask about fees before you book. Many suppliers charge extra for small loads below a set volume, and for waiting time once a truck has been on site past a free period. For example, one major Australian supplier’s published fee schedule applies a charge to loads under 4 m³ and charges for waiting time after the first 45 minutes. Amounts and thresholds differ between suppliers and regions, so ask your local plant for their current rates.

On a split pour, or when you’re unsure, it’s often better to hold back a small part of the order and call the last load once you can see what’s left to fill. Talk to the plant about how late you can call it.

A quick checklist before you order

• Every measurement converted to metres
• Odd shapes split into simple rectangles and cylinders
• Thickened edges, beams and steps worked out separately and added
• Depth checked in several places, not just one
• Waste allowance added and the total rounded up
• Premix yield taken from the bag or data sheet
• Mix, access, pump and timing confirmed with the plant
• Small load and waiting fees asked about up front

If you’d rather not do the sums by hand on site, our free Site Concrete Calculator works out slab, footing and post hole volumes for you.